1. Field of the Invention
This invention relates to gas nozzles, and more particularly to such nozzles as useful in refining molten metals, and to a method of manufacturing such nozzles and the use of such nozzles.
2. Description of the Prior Art
Molten metal may be refined by blowing gas through nozzles disposed at the bottom of a converter. This practice is carried out in bottom blowing converters, top-bottom blowing converters, or the A.O.D. process (Argon Oxygen Decarburization).
The nozzle, which is disposed at the bottom or at a wall of such converters, usually comprises a refractory structure which is positioned at the bottom of the converter, a plurality of passages made in the refractory structure, a gas storage area formed at the lower part of the refractory structure for keeping constant the amount of gas flowing into the passages, and a gas pipe for supplying gas to the nozzle. The gas is blown through a gas pipe connected to a gas source into the converter via the gas storage area and through each of the passages.
When blowing gas into the converter via the nozzle of the mentioned prior art structure, the gas directly attacks the refractory structure, depending upon the relation therebetween, and causes deterioration of the refractory structure, (for example using a refractory material of MgO.C brick and CO.sub.2 gas), and thus resulting in shortening of the life of the refractory structure. When the refractory structure is caused to become thin due to deterioration or losses due to action of the molten metal, and if the refractory structure is directly affected at its bottom, the nozzle may become broken by the pressure of the gas. Thus, since the life of the nozzle is extremely short, and the above problems exist, the range of gas pressure to be used cannot be made large.
In the prior art, one or more of the following methods have been used in producing gas nozzles having refractory structures which can be positioned at the bottom of a vessel supporting molten metal.
(1) The grain sizes of raw materials of the refractory structure may be controlled so that a porous refractory structure is produced by the forming and baking process.
(2) A burnable material may be used, together with the refractory raw material effected with grain size control, wherein the burnable material and refractory material are mixed, formed and the burnable material subsequently burned to produce a porous refractory structure.
(3) A plurality of narrow, lengthy pieces of paper or wood may be buried in a body of refractory structure, and subsequently removed to form holes running in straight lines from the exposed working face contacting the molten metal to the read end. For an example of such method, see Japanese Laid Open Patent Specification No. 42,531/72.
The above mentioned conventional manufacturing methods all have deficiencies and problems. For example, in the above mentioned methods (1) and (2), it is difficult to make gas flow in one direction; instead, the flowing directions are at random. Thus, it is necessary to seal the side face of the nozzle, other than the gas jetting face and the gas supplying face, with non-porous refractory material or sealing material. The conventional methods make the refractory structure porous by controlling the grain sizes. Thus, the amount of jetting gas is restricted. A large amount of air permeability cannot be obtained. Furthermore, since the sizes and shapes of the holes for passage of gas are varied, the gas jetting pressure is not constant. Thus, losses or damage caused by molten metal is large. Moreover, because the entire refractory structure is porous, long life cannot be obtained.
The gas blowing refractory structure made by the above method (3) seemingly has solved the above problems, but in actuality, other problems and deficiencies have been found to exist. For example, Paper or wood is generally low in strength and is deformed during processing. Thus, using this prior technique, it is difficult to provide accurate predetermined diameters of the holes used for passing gas, and cracks are caused to be formed in the refractory body when high pressure is effected during gas passage.
Furthermore, the burning material generates unwanted volatile matter or gas. Cracks are created during burning and burnable leftovers often remain in the refractory structure. Perfect opening of gas passageways cannot be obtained. It is especially difficult to produce nozzles of required sizes (e.g. large lengths) to be used at the bottom of the converter.
Moreover, temperatures of operation should be higher than the burning temperature to form narrow holes. Also, the above methods cannot be applied to non-burnt refractory structures, or non-burnt castable cast products.
Due to these problems, and deficiencies, refractory structures of the prior art are limited with constant holes being formed in only limited areas, with jetting of large volumes of gas.
Furthermore, as top blow converters have become large scaled, gas is blown from the bottom of the converter to circulate molten metal. This practice is called top-bottom blowing. For such bottom blowing nozzles, SUS pipes or porous bricks are employed.
With respect to the nozzle of the pipe, the diameter is generally from 5 to 20 mm, and the flow rate of gas should be higher than mach, and if the flow rate is lower than mach, the nozzle may tend to become clogged. This is a necessary condition while the converter supports the molten metal. The upper limit is a flow rate which produces a pressure of around 30 kg/cm.sup.2, in view of the pressures which can be used industrially. Thus, the range between the two forms what might be termed the control range for the bottom gas flowing. That is to say, the lower limit of flow rate of bottom blowing gas is determined by the flow rate at which there occurs nozzle clogging and the upper limit depends upon the pressure limit of the facility. The range between the lower limit and the upper limit of gas blowing rate is around 2 to 3 times, that is, the upper limit is 2 to 3 multiples of the flow rate at which clogging occurs.
In view of the metallurgical phase, when the bottom gas flow rate is increased, reaction of slag and metal is made active and dephosphorization is accelerated. In low carbon material (C=less than 0.04%), P content is lowered as the amount of gas increases. However, in high carbon materials (C=more than 0.4%), agitation between the slag and metal is too strong and oxidation potential in the steel and the slag is lowered, to extremely deteriorate dephosphorization. Thus, it is seen that the bottom gas flow rate requires 0.005 to 0.011 Nm.sup.3 /min.multidot.T, for providing preferable dephosphorization in the refinging range of C=0.04 to 0.4%. See for example, FIG. 7, which depicts such relationship.
However, in pipe nozzles, since the gas controlling range of flow rate is narrow, the effect is not preferable in the high carbon range with respect to the bottom gas flow rate. When obtaining maximum effect in the low carbon range, the effect in the high carbon range is inferior with the above relevant bottom gas flow rates. When obtaining maximum effect in the high carbon range, the effect in the low carbon range is similarly inferior using the above relevant gas flow rates. Thus, when selecting the gas flow rate (e.g. 0.10 Nm.sup.3 /min.multidot.T) the lower limit of gas flow rate is about 0.03 to 0.05 Nm.sup.3 /min.multidot.T, and dephosphorization is accelerated by lowering C at the end point to low C. Consequently, the yield of molten steel is inevitably lowered and the basic unit of alloy is heightened, and further, since the gas should not be stopped, the basic unit of the bottom blow is restricted.
In order to improve the above deficiencies, of prior pipe nozzles, there has been proposed, a porous nozzle of porous brick which controls the gas flow rate from 0. The porous nozzle is formed by controlling grain sizes within a certain range, and making permeability less than about 100 microns. If the gas blow is stopped while the steel is held in the converter, the steel hardly penetrates into the porous nozzle, and some of the above problems are resolved. However, not all problems are solved. Since the gas runs into crystalline grains of the refractory structure in the porous nozzle, resistance is extremely large there and gas pressure should be kept high to control the gas. Such high gas pressure will inevitably cause damage to the refractory structure of the nozzle. Thus, in conventional nozzles, the upper limit of gas pressure was found to be about 30 Kg/cm.sup.2. See for example, FIG. 6, wherein the lower limit due to clogging is depicted together with the upper limit due to facility breakdown.